Look, if "pied" is singular so they use son, like: son pied. But if it s plural so they use ses, like: ses pieds.
For using possessives, first u should see for who you will use it? First, second or third person (singular or plural), then see if the name is masculine or feminine or plural. Check these examples.
............... chaussures (je) - "chaussure" is plural and you should use possessives for "je" which are "mon", "ma", "mes". mes is the answer so you should say "mes chaussures.
............. livre (tu) - "livre" is masculine, plural and you should use possessives for "tu" which are "ton", "ta", "tes". ton is the answer so you should say "tonlivre.
Hope it was helpful, by the way ready for answer your questions